In broad perspective this program of research is directed toward a further understanding of factors which influence social behavior in animals. Especially emphasized are the neural and endocrine activities that may both determine and result from social interaction. The program is based upon recognition of the fact that the internal milieu of certain animals is highly responsive to changes in the social environment. Conversely, changes in the internal state of the organism can lead to significant alterations in social behavior. Consideration of these reciprocal relationships forms the basis for the studies with which we are immediately concerned. These experiments have involved investigation of some of the physiological and behavioral changes which take place during the period of pair formation in the ring dove (Streptopelica risoria). Based on these studies, three lines of research have emerged: (1) Determination of the effects of social behavior on hormone changes in a mate; (2) delineation of neural and endocrine factors controlling changes in male behavior during the breeding cycle; and (3) definition of the determinants of mate selection and fidelity.